Works great for me! Rated 5 out of 5 stars
I am using Firefox 11.0 and this add-on works fine for me.
Though, it does not change the bookmarks toolbar from its horizontal state into a vertical position or adds another toolbar that is vertical. (At least not in FF 11.)
No, but it fixed the "Vertical Toolbar" add-on by Gomita. If you placed your bookmarks into that vertical toolbar, they kept beeing displayed horizontal instead of changing into a vertical order, too.
Now, with this addon right here they are finally displayed vertical inside of the "Vertical Toolbar".
Which is just what I was looking for.